Page 241 - Applied Petroleum Geomechanics

236 Applied Petroleum Geomechanics

Figure 7.2 Normal pore pressure profiles for pore fluids of fresh water and brine
compared to overburden stress in the subsurface.

greatly in salinity; therefore, formation pore pressure also changes with
salinity, as shown in Table 7.1.
Generally, the density of water in sedimentary basins varies from 1.0 to
3
3
1.08 g/cm (for the saturated saltwater, r w ¼ 1.2 g/cm ), for example:
3
Gulf Coast (Mississippi delta): r w ¼ 1.07 g/cm ;
3
Rocky Mountains and the North Sea: r w ¼ 1.02 g/cm ;
3
Nigeria delta: r w ¼ 1.08 g/cm .
7.1.3 Overpressure and underpressure
When pore pressure is lower or higher than the hydrostatic pressure, it is
abnormal pore pressure. Pore pressure that is greater than the hydrostatic
pressure is termed overpressure or excess pressure (Fig. 7.3). Likewise, pore
pressure that is less than the hydrostatic pressure is termed underpressure, or
subnormal pressure. Underpressure and overpressure together are
composed of the classification of abnormal pressures. The terms of
abnormal pressure and overpressure are sometimes used interchangeably